In Java(tm), null is not a keyword, but a special literal of the null type. It can be cast to any reference type, but not to any primitive type such as int or boolean. The null literal doesn't necessarily have value zero.
Add a jump rope to the motion and you will be skipping rope. The skipping moves won't bulk you up, but they can tone the muscles in your thighs, calves and butt -- your quadriceps, hamstrings, gluteals, iliopsoas, gastrocnemius and tibialis anterior.
Jumping Jacks can be a fun exercise, and can be easily learned. It  uses calve muscles, hip/shoulder abductors, shoulder/his adductors,  and the core.